Using a Yubikey for SSH on macOS
SSH 8.2 introduced support for using any U2F key in place of a private key file. Using it on macOS with full support for ssh-agent
is a bit more complex.
Generating the keys
-
You must choose between
ed25519-sk
andecdsa-sk
. Tryed25519-sk
(Options 1 or 3) first. If it does not work due to device incompatibilities, fall back onecdsa-sk
(Options 2 or 4) -
You must choose if you want to store the key handle as a resident key on the device. If you want to, use options 1 or 2. If not, use options 3 or 4.
A U2F attestation requires a key handle to be sent to the device. When generating the key,
ssh-keygen
will create private and public key files that look similar to normal ssh key. The private key file is actually a key handle that cannot be used without the hardware token, however, the hardware token can also not be used without the key handle.A resident key solves this problem by storing the key handle on the device. However, your key may or may not support it and only a limited number of resident keys may be stored on a device. Additionally, it may reduce the security of your ssh key as they could use it if they steal the hardware device. For this reason, a good pin is important.
It is your choice whether to use a resident key. If you do, you can load it directly to the ssh-agent using
ssh-add -K
, or write the key handle and public key to disk usingssh-keygen -K
ssh-keygen -t ed25519-sk -O resident # 1
ssh-keygen -t ecdsa-sk -O resident # 2
ssh-keygen -t ed25519-sk # 3
ssh-keygen -t ecdsa-sk # 4
Updating SSH
SSH v8.2 is required to use a security key. Install it with brew.
brew install openssh
You can specifiy the path to the private key handle in your ssh config. Otherwise, you can configure the ssh-agent.
ssh-agent on macOS
To be used with a security key, the ssh-agent must be on v8.2, which the system default is not.
First, disable the macOS default ssh-agent for your user.
launchctl disable user/$UID/com.openssh.ssh-agent
Next, add a new launchd service for your ssh-agent. Add the following file to ~/Library/LaunchAgents/com.zerowidth.launched.ssh_agent.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.zerowidth.launched.ssh_agent</string>
<key>ProgramArguments</key>
<array>
<string>sh</string>
<string>-c</string>
<string>/usr/local/bin/ssh-agent -D -a ~/.ssh/agent</string>
</array>
<key>RunAtLoad</key>
<true/>
</dict>
</plist>
And load it with launchctl load -w ~/Library/LaunchAgents/com.zerowidth.launched.ssh_agent.plist
.
In your .bashrc
or .zshrc
, set SSH_AUTH_SOCK="~/.ssh/agent"
This plist was created using the launchd plist generator over at zerowidth. It runs the command /usr/local/bin/ssh-agent -D -a ~/.ssh/agent
. -D
prevents ssh-agent from forking, and -a ~/.ssh/agent
directs the agent to create a socket file at that location that is referenced in $SSH_AUTH_SOCK
.